In order to purify NOx in the exhaust gas of a diesel engine, for example, an SCR system that uses a selective reduction catalyst (SCR) is developed as an exhaust gas purification system (for example, refer to Japanese Patent Application Laid-open No. 2000-27627 and Japanese Patent Application Laid-open No. 2005-83223).
In the SCR system, urea is used as a reductant, an aqueous urea solution is supplied upstream of the exhaust gas of the SCR apparatus, the urea is hydrolyzed by heat of the exhaust gas to generate ammonia, and NOx is reduced by the ammonia on an SCR catalyst to achieve purification.
The aqueous urea solution is stored in a tank, and an injection quantity of the aqueous urea solution injection is calibrated in accordance with an engine speed—load so that a target purification rate can be achieved at a predetermined concentration (for example, 32.5%).
Replenishing the aqueous urea solution may be problematic since dilution of the aqueous urea solution by water may cause a decline in purification performance and, conversely, accidentally adding an aqueous urea solution with a high concentration may cause an ammonia slip due to an oversupply of urea or clogging of an injection nozzle due to urea deposition.
In addition, an increase in concentration due to water evaporation in the urea tank or an increase in concentration due to contamination by a foreign liquid such as a saline solution may conceivably cause a failure in an injection system or the SCR apparatus.
An urea concentration sensor such as that described in Japanese Patent Application Laid-open No. 2007-47006 is provided in the aqueous urea solution tank. The sensor detects and diagnoses a concentration of the aqueous urea solution, and judges that the concentration is abnormal when the concentration is lower or higher than a predetermined value, whereby injection of the aqueous urea solution is prohibited and a warning is issued to an operator to prompt replacement of the aqueous urea solution inside the tank with a normal aqueous urea solution.